Sulfite spent liquors are produced as a byproduct in the production of wood pulp via the well-known sulfite process. Sulfite spent liquors comprise undissolved wood solids, lignosulphonates, hydroxy acids as aldonic acids and hexose and pentose sugars, as well as various cooking chemicals introduced in the pulping process. In the past, these liquors were either concentrated, dried or desugarized to be used as lignosulphonate products for example in feeds or discarded, e.g. by discharging these into the environment or by drying the liquor and burning the remaining material and recovering cooking chemicals. Both of these practices for disposal of the sulfite spent liquors have environmental disadvantages, as these tend to pollute the environment with potentially damaging chemicals such as sulfur compounds, the sulfite spent liquor generally having a high sulfur content. Accordingly, there has in past been much focus on disposal of these liquors in an improved manner.
A key development in the disposal of sulfite spent liquors was the realization that the liquors, generally considered to be an invaluable product, include commercially valuable components including xylose. Xylose is an aldopentose, a monosaccharide containing five carbon atoms and including an aldehyde functional group. It has the chemical formula C5H10O5.
Xylose is particularly valuable because it can easily be converted into xylitol, a five-carbon sugar alcohol which has applications as a sugar substitute. Xylitol is roughly as sweet as sucrose, but has a caloric value of 2.4 kcal/g compared to the value of sucrose, 4 kcal/g. Furthermore, xylitol contains no net effective carbohydrates, whereas sucrose contains 4 grams carbohydrate per teaspoon. Furthermore, xylitol has virtually no aftertaste, and is generally considered to be safe for diabetics and individuals with hyperglycaemia, because sugar-alcohols have less impact on a person's blood sugar than regular sugars. The use of xylitol as a sweetener is considered advantageous because xylitol is a “tooth friendly” sugar; in addition to not encouraging tooth decay in the same way as sucrose and glucose, xylitol actively aids repair of minor cavities caused by dental caries.
For the above reasons, xylitol is a commercially valuable sugar which is finding increasingly many applications in the manufacture of e.g. foodstuffs and sweets. As xylitol can easily be prepared from xylose, there is a great interest in finding ways to extract xylose from sulfite spent liquors in the most efficient manner possible. Accordingly, scientists have for several years been trying to find the best possible separation process whereby xylose can be isolated in high purity and high yield, in a manner which is fast, efficient, convenient and cheap.